Why It Matters
Insulation bore size must match the pipe outside diameter (OD) — not the nominal pipe size. A snug fit is essential: gaps reduce thermal performance and allow vapour ingress on cold pipes; compressed insulation is thinner than specified and may not keep the surface above dew point. Measure the actual OD of the bare pipe before ordering.
Three Measurement Methods
Method 1: Digital Caliper (Most Accurate)
- Open the caliper jaws wider than the pipe
- Place jaws across the widest point of the pipe
- Close gently until they touch the pipe surface
- Read the measurement and round up to the nearest standard bore size
Accuracy: ±0.1mm. Best for professional installations.
Method 2: Tape Measure (Quick Estimate)
- Wrap a tape measure around the pipe circumference
- Note the circumference (e.g. 69mm)
- Divide by π (3.14) to get the diameter: 69 ÷ 3.14 = 22mm OD
- Round up to the nearest standard bore size
Accuracy: ±1–2mm. Good for accessible pipes when a caliper isn't available.
Method 3: Pipe Gauge
Wrap the gauge around the pipe and read the diameter directly. Accuracy ±0.5mm. Useful for professional installers doing multiple measurements on site.
OD to Bore Size Reference
| Measured OD | Use This Bore Size |
|---|---|
| 15–16mm | 15mm or 18mm |
| 18–20mm | 18mm or 22mm |
| 22–23mm | 22mm |
| 26–29mm | 28mm |
| 33–36mm | 35mm |
| 40–43mm | 42mm |
| 46–50mm | 48mm |
| 52–56mm | 54mm |
| 58–62mm | 60mm |
| 74–78mm | 76mm |
If your measurement falls between sizes, round up to the next size for a snug fit.
Pipe Type Quick Reference
| Pipe Type | Nominal Size | Actual OD | Bore Size to Order |
|---|---|---|---|
| Copper (UK) | 15mm | 15mm | 15mm |
| Copper (UK) | 22mm | 22mm | 22mm |
| Copper (UK) | 28mm | 28mm | 28mm |
| Steel (imperial) | 1/2" | 21.3mm | 22mm |
| Steel (imperial) | 3/4" | 26.9mm | 28mm |
| Steel (imperial) | 1" | 33.7mm | 35mm |
| MDPE | 15mm | 16–18mm | 18mm or 22mm |
| MDPE | 20mm | 20–22mm | 22mm |
| MDPE | 25mm | 25–28mm | 28mm |
Common Mistakes
- Using nominal pipe size — always measure the actual OD. Nominal size ≠ actual OD, especially for steel and MDPE.
- Measuring internal diameter — always measure outside diameter.
- Measuring over existing insulation or paint — measure the bare pipe.
- Not measuring at multiple points — pipes can be slightly oval; take two or three readings.
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